Methyl parathion (MP) is an organophosphate pesticide used in agriculture, although quite often illegally used indoors to contain insects. The present study was planned to investigate the effects of MP on rat testis. Adult male Wistar rats (13-14 weeks) were treated with MP as follows. Experiment 1-0, 1.75, 3.5 or 7 mg/kg ip for 5 days and sacrificed on Day 14; experiment 2 and 3-0, 0.5, or 1 mg/kg ip for 12 days, and sacrificed on Days 130 and 77, respectively; experiment 4-0, 0.75, or 1.5 mg/kg ip for 25 days, and sacrificed on Day 17; experiment 5-0 or 3.5 mg/kg po for 25 days, and sacrificed on Day 17, after the last exposure. MP decreased the body weight and the testis weight in experiments 4 and 5 (p< 0.05-0.001) due to decreased food intake and tubular atrophy respectively. MP increased the intra-testicular testosterone level and decreased the LH level in experiments 4 and 5. The seminiferous epithelium showed sloughing of germ cells, vacuoles, focal necrosis, and formation of multinucleated giant cells, cellular degeneration (nuclear pyknosis, halo appearance and shrinkage of nuclei) and tubular atrophy, especially in experiment 4. The degree of testicular damage was higher in experiment 4> 5> 1> 3> 2 indicating more effect of prolonged ip treatment. Homogenization-resistant spermatid count was decreased in experiments 1, 4 and 5, and MP also decreased the tubular diameter, and epithelial height (p< 0.05-0.001). Incidences of stage XIV tubules, number of meiotic figures and elongating spermatids were also decreased, whereas the incidence of tubules showing epithelial sloughing increased (p< 0.05-0.001). We conclude that MP is a reproductive toxicant in male rats which causes significant testicular damage in the testis.